Recent development in filter conversion techniques has enabled a very efficient QMF representation of a time-domain filter. In general any FIR filter (FIR=Finite Impulse Response) in the time-domain can be converted into a set of complex filters each corresponding to a specific subband in the QMF. Hence, the filtering can take place in the complex QMF domain, similarly to how filtering can be performed using FFTs (FFT=Fast Fourier Transformation). Even so, the computational complexity of the QMF domain representation and implementation of the filtering can be substantial, for instance in the case of filters having a long impulse response in the time-domain.
Furthermore, recent development in audio coding has made available the ability to recreate a multi-channel representation of an audio signal based on a stereo (or mono) signal and corresponding control data. These methods differ substantially from older matrix based solution such as Dolby Prologic®, since additional control data is transmitted to control the re-creation, also referred to as up-mix, of the surround channels based on the transmitted mono or stereo channels.
Hence, such a parametric multi-channel audio decoder, e.g. MPEG Surround, reconstructs N channels based on M transmitted channels, with N and M are possible integers, wherein N>M, and the additional control data. The additional control data represents a significant lower data rate than transmitting all the N channels, making the coding very efficient while at the same time ensuring compatibility with both M-channel devices and N-channel devices.
These parametric surround coding methods usually comprise a parameterization of the surround signal based on IID (Inter channel Intensity Difference) and ICC (Inter Channel Coherence). These parameters describe power ratios and correlation between channel pairs in the up-mix process. Further parameters also used in conventional approaches comprise prediction parameters used to predict intermediate or output channels during the up-mix procedure.
Other developments in audio coding have provided means to obtain a multi-channel signal impression over stereo headphones. This is commonly done by downmixing a multi-channel signal to stereo using the original multi-channel signal and the so-called HRTF (Head Related Transfer Functions) filters. It has been shown in conventional approaches that the parametric multi-channel audio decoder can be combined with a binaural downmix algorithm making it possible to render a multi-channel signal over headphones without the need for first re-creating the multi-channel signal from the transmitted downmix signal, and subsequently downmixing it again by means of the HRTF filters. This is accomplished by combining the HRTF filters into four filters as a function of the parametric multi-channel representation. As a consequence, the four filters describe as a function of the parametric multi-channel representation how the stereo signal (two channels) used as an input for the multi-channel representation will be combined or mixed to achieve the resulting binaural or stereo output signals (two channels). So each of the four filters relate to one of the two input signals with respect to the two output signals. However, the HRTF filters can be quite long in order to nicely model room characteristics, and therefore the computational complexity of filtering the four HRTF filters in the QMF domain can become significant.